Host detection of microbial pathogens triggers a chain of events that results in recruitment and differentiation of T lymphocytes with functions suited to restrain offending microorganisms. Th17 cells, a newly defined subset of CD4+ T helper cells, secrete signature cytokines interleukin (IL)-17A, IL-17F, and IL-22. Th17 cells have been shown to play an important role in maintenance of mucosal integrity and immunity. On the other hand, dysregulated Th17 responses result in autoimmunity both in mice and in humans. Another subset of CD4+ T cells, the regulatory T cells (Tregs), suppresses effector T cell responses and prevents their potentially pathogenic effects. The balance between Th17 and Treg cells is influenced by the local cytokine milieu and is achieved through the concerted action of transcription factors to maintain immune homeostasis. ROR?t and Foxp3 have been shown to be the key transcription factors in specifying differentiation programs of Th17 and Treg lineages, respectively. Our recent data suggest that Th17 and Treg fate decision can be determined by the balance of ROR?t and Foxp3, at least in part through physical interaction. Recently, it has been shown that the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor best known to mediate the effects of environmental toxins (e.g. dioxin), is an important transcription factor in the Th17 lineage, especially for IL- 22 expression. AhR also regulates Foxp3 expression, thus likely influencing the balance of Th17 and Treg cell differentiation. However, little is known about the molecular mechanisms of action of AhR in these processes. The objective of this proposal is to examine how the interaction of transcription factors will impact Th17 and Treg cell differentiation and function. Proposed are experiments to investigate the interplay among AhR, ROR?t, and Foxp3. A combination of biochemical, molecular, and genetic approaches will be employed to study the molecular action of AhR in determining Th17 cell differentiation program, the cross-regulation of AhR and Foxp3 during Th17 and Treg differentiation, and the functional significance of AhR activation in a ligand- independent and cell-type specific manner in vivo. These experiments will offer an exciting opportunity to provide fresh mechanistic insights into how AhR coordinates with ROR?t and Foxp3 to program Th17 and Treg cell differentiation, and shed light on AhR function in modulating Th17 and Treg balance in the physiological steady state or during microbial infection. A better understanding of the mechanism by which AhR regulates Th17 and Treg cell differentiation will be informative toward the identification of new targets for treating human infectious and autoimmune diseases.